This invention relates generally to electrical propulsion systems for diesel electric vehicles such as locomotives equipped with alternating current (AC) electric traction motors and, more particularly, to a method and apparatus for using a traction inverter to supply AC electric power for non-traction motor applications.
In a conventional diesel electric locomotive, a thermal prime mover (typically a 16 cylinder turbo-charged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of alternating current (AC) traction motors whose rotors are drivingly coupled through speed reducing gearing to respective axle wheel sets of the locomotive. The generator typically comprises a main three-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the diesel engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in three-phase armature windings on the stator of the alternator. These voltages are rectified to produce a controlled amplitude DC voltage and then applied to one or more PWM (pulse width modulation) inverters which control the effective frequency of alternating current to be supplied to the armature windings of the AC traction motors. The effective AC excitation frequency produced by the inverters controls the speed of the AC motors with power being controlled by pulse width modulation of the AC waveform.
Prior to starting of these very high horsepower diesel engines, it is common practice to initiate a pre-lube of the engine by actuating a lubrication drive pump to force lubricant through the engine. The pump is driven by a three-phase, AC electric motor. The motor is powered from the vehicle battery using a DC-to-DC converter to obtain voltage magnitude followed by a three-phase inverter to convert the DC voltage to a fixed frequency, three-phase excitation for the pump motor. This system has several disadvantages, most notable of which is the added cost of a power DC-to-DC converter and a three-phase power inverter. A secondary disadvantage is that a single failure of either the converter or inverter will preclude operation of the locomotive. Accordingly, it would be advantageous to provide a method and apparatus for powering a pre-lube system which does not require dedicated power converters and inverters.
At initial start-up of the locomotive after the pre-lube operation discussed above, the on-board battery is also used to provide electrical power for cranking of the diesel engine. Typically, the battery is connected to supply DC power to a cranker inverter and the inverter is operated to convert the DC power to controlled frequency AC power. The cranker inverter is switched into circuit with the synchronous generator and the generator is operated as a motor to turn the output shaft of the engine for cranking.
The cranker inverter is typically a current-fed, third harmonic, auxiliary impulse inverter for supplying the variable frequency alternating current to the 3-phase stator windings of the rotatable synchronous generator that is used to start or "crank" the diesel engine, i.e., the generator is operated as a motor and the rotor of the generator is coupled to the crankshaft of the engine to rotate the crankshaft for starting. Initially the output torque of the rotor (and hence the magnitude of current in the stator windings) needs to be relatively high in order to start turning the crankshaft. As the rotor accelerates from rest, less torque (and current) will be required, while the fundamental frequency of load current increases with speed (revolutions per minute). In its cranking mode of operation, the inverter supplies the machine with current of properly varying magnitude and frequency until the engine crankshaft is rotating at a rate that equals or exceeds the minimum speed at which normal running conditions of the engine can be sustained.
In essence, a third harmonic auxiliary impulse commutated inverter comprises six main unidirectional conduction controllable electric valves, such as thyristors, that are interconnected in pairs of series aiding, alternately conducting valves to form a conventional 3-phase, double-way, 6-pulse bridge between a pair of DC terminals and a set of three AC terminals. The DC terminals of the bridge are adapted to be connected to the on-board locomotive battery. The AC terminals of the aforesaid bridge are respectively connected to the different phases of a 3-phase electric load circuit which typically comprises the star-connected 3-phase stator windings of the generator.
To supply the load circuit with 3-phase alternating current, the six main valves of the inverter are cyclically turned on (i.e., rendered conductive) in a predetermined sequence in response to a family of "firing" signals (gate pulses) that are periodically generated in a prescribed pattern and at desired moments of time by associated control means. To periodically turn off the main valves by forced commutation, the inverter is provided with an auxiliary circuit comprising a pre-charged commutation capacitor and at least seventh and eighth alternately conducting unidirectional controllable electric valves that are arranged to connect the capacitor between the neutral or common point of the 3-phase AC load circuit and either one of the DC terminals of the bridge. Once the engine has been "cranked", the inverter is disconnected from the system and is not used until the engine is again started. Accordingly, it would be desirable to eliminate the need for a separate inverter for cranking on the engine and further desirable to provide a controllable frequency power source that is less complicated than the third harmonic, auxiliary impulse commutated inverter.